format_literal.cpp

struct formater
{
    formater(const char* str, size_t size)
      : format(str, size)
    {
    }
    
    template<class ...T>
    std::string operator()(T&&... s) const
    {
        char str[10000] = {0};
        sprintf (str, format.c_str(), std::forward<T>(s)...);
        return std::string(str);
    }
    
    std::string format;
};

formater operator"" _format ( const char* str, size_t size )
{
    return formater{str, size};
}

int main()
{
    auto s = "Hello %s world%s"_format(",", "!");
  std::cout << s ;
}